Leave your panels flat in summer: tilting gains under 10% at midday across most of the US. Midwinter is a different story: at midday a tilted panel collects 50% to 250% more direct sun than a flat one, depending on how far north you're parked. Clouds and diffuse light shrink the real-world gain, but it stays large. The table below shows exactly where the line is.
The figures are the midday direct-sun gain from tilting to the optimal angle instead of leaving panels flat on the roof, computed from the sun's noon elevation at each latitude.
| Latitude | Midsummer | Spring and fall | Midwinter |
|---|---|---|---|
| 25° (Miami, south Texas) | +0% | +10% | +51% |
| 30° (Houston, New Orleans) | +1% | +15% | +68% |
| 35° (Albuquerque, Memphis) | +2% | +22% | +91% |
| 40° (Denver, Philadelphia) | +4% | +31% | +123% |
| 45° (Minneapolis, Portland OR) | +8% | +41% | +172% |
| 50° (southern England, Calgary) | +12% | +56% | +250% |
These are midday, clear-sky ceilings: clouds, diffuse light, and morning and evening hours pull real daily gains below them. The pattern is what matters, and it holds: tilting barely pays in summer and pays enormously in winter.
Tilt when you'll sit in one spot for two or more days, and almost always from October to February. Skip it for one-night stops in summer: the gain won't cover the setup time, and a panel lying flat can't blow over.
The calibrated optimum for where you are, which runs from 27° on the Gulf coast to 42° at the Canadian border. That is shallower than your latitude, by up to 8° once you are that far north. Add 14° to 21° for midwinter, more the further north you are, and take off 17° to 25° in high summer, less the further north you are. Camped somewhere for a while? Look up the nearest town for its exact monthly table, or use your zip code or UK postcode.
Nose the rig so your tilted panels face true south (true north in the southern hemisphere). Tilting along the roof's long axis is easiest, so a spot where south is off your driver's side or curb side, whichever your legs lift toward, saves you re-parking.